Seismic waves generated artificially have been used for more than 50 years to perform imaging of geological layers. During seismic exploration operations, vibrator equipment (also known as a “source”) generates a seismic signal that propagates in the form of a wave that is reflected at interfaces of geological layers. These reflected waves are received by geophones, or more generally receivers, which convert the displacement of the ground resulting from the propagation of the waves into an electrical signal which is recorded. Analysis of the arrival times and amplitudes of these waves make it possible to construct a representation of the geological layers on which the waves are reflected.
FIG. 1 depicts schematically a system 100 for transmitting and receiving seismic waves intended for seismic exploration in a land environment. The system 100 comprises a source 102 consisting of a vibrator operable to generate a seismic signal, a set of receivers 104 (e.g., geophones) for receiving a seismic signal and converting it into an electrical signal and a seismic data acquisition recorder system (recorder system) 106 for recording the electrical signals generated by the receivers 104. The source 102, the receivers 104 and the recorder system 106 are positioned on the surface of the ground 108. FIG. 1 depicts source 102 as a single vibrator but it should be understood that the source may be composed of several vibrators, as is well known to persons skilled in the art. System 100 further includes vehicle 122a, for housing the source 102, and vehicle 122b for housing recorder system 106, as well as antennas 124 for communications between vehicles 122a,b (and source 102) and receivers 104. The receivers 104 are interconnected by cables 126 and connected to recorder system 106. Antennas 124 on receivers 104 can communicate data from receivers 104 to recorder system 106, as can cables 126. Furthermore, in operation, vehicle 122a is generally not static, but generates transmitted waves in different locations for the GAI.
In operation, source 102 is operated so as to generate a seismic signal. This signal propagates firstly on the surface of the ground, in the form of surface waves 110, and secondly in the subsoil, in the form of transmitted waves 112 that generate reflected waves 114 when they reach an interface 115 between two geological layers. Each receiver 104 receives both a surface wave 110 and a reflected wave 114 and converts them into an electrical signal, which signal thus includes a component associated with the reflected wave 114 and another component associated with the surface wave 110. Since system 100 intends to image the subsurface regions 116 and 118, including a hydrocarbon deposit 120, the component in the electrical signal associated with the surface wave 110 is undesirable and should be filtered out.
In addition to “straight” surface waves 110, i.e., those that proceed more or less linearly from source 102 to receivers 104, there are other surface wave types that also present problems associated with generating an accurate image of the desired subsurface strata. For example, so-called scattered waves are surface waves that reflect off of objects or boundaries in or around the surface. Scattering is caused by, for example, large underground rocks, mineral seams, and other objects of that nature, collectively referred to here as “scatterers”. A scatterer, therefore, acts like another point source or generator of seismic energy, but one which was not intended to be used to image the subsurface. In the presence of scatterers, therefore, 3D frequency-wavenumber (fk) filtering, also referred to as fan filtering, can be a useful tool to filter out scattered waves if a so-called a full three dimensional (3D) source-receiver pattern is used, hereafter referred to as “Full 3D”.
FIG. 2A illustrates an example of a Full 3D source-receiver pattern 200 from a top view perspective. Therein, a single source 202 is disposed at the center of a field of receivers 204 (only a few of which are numbered to avoid obscuring the drawing). When the single source 202 generates a seismic signal (shoots the area), reflected wave energy will be received by the receivers 204 within the covered area 206. The receivers 204 will also receive surface waves from three exemplary scatterers 208. As mentioned above, these scattered waves can be removed by using 3D fk filtering. However, it has been determined that a significant part of the scattered wave energy cannot be filtered out using 3D fk filtering techniques in seismic exploration systems which use a cross-spread source-receiver pattern instead of a Full 3D design (see, for example, “3D geometry, Velocity Filtering and Scattered Noise,” Meunier J., 69th Annual International Meeting, SEG, Expanded Abstracts, 1216-1219 [1999]).
To better understand why this is so, compare the Full 3D source-receiver pattern (Full 3D pattern) 200 with a cross spread pattern source-receiver (cross spread pattern) 300 illustrated in FIG. 3A which is intended to cover the same area 206. Instead of using one source or shotpoint 202 and a large number of receivers 204 as in the Full 3D pattern 200, the cross spread pattern 300 uses more shotpoints 302 and fewer receivers 304 arranged in an intersecting, cross pattern. It will be appreciated by those skilled in the art that, in some cases, it may be more cost effective to deploy a seismic acquisition system using the cross spread pattern 300 rather than the Full 3D pattern 200, since the cross spread pattern requires the deployment of many fewer receivers. The same three scatterers 208 are also present in this area 206 such that the comparison of the scatterer effects for both Full 3D pattern 200 and cross spread pattern 300, discussed below, is meaningful.
In order to compare received signal energy by the different source-receiver patterns 200 and 300, consider a Full 3D design and a cross spread design having the same covered area 206 and which also generate the same number of seismic traces. For example, suppose that the Full 3D source-receiver pattern 200 includes a single source 202 and 9801 receivers 204 for a total of 9801 traces, i.e., 1×9801. Similarly, suppose that the cross spread source receiver pattern 300 includes 81 sources 302 and 121 receivers 304, which will also generate 9801 traces (i.e., 121×81). While these source-receiver patterns cover the same area, and generate the same number of seismic traces, it will be seen below that the signal energy received for comparable traces will be different due to the difference in the source-receiver patterns.
More specifically, compare the time slice of a seismic trace illustrated in FIG. 2B for the Full 3D system 200 with the time slice illustrated in FIG. 3B of a comparable seismic trace for the cross spread system 300, which seismic traces have been generated using synthetic data. In this context, “comparable” seismic traces refer to traces from each system which image the same point in the subsurface, i.e., the common midpoint (CMP). Each time slice shows four energy arrival patterns, one (210 and 310) associated with the primary reflected wave from the source and three associated with scattered waves 212, 214, 216 and 312, 314, 316 generated by the three scatterers 208. It can be observed that the largest patterns 210 and 310, which correspond to the primary wave arrival, have the same circular pattern for both source-receiver patterns 200 and 300's time slices.
However while the three scatterer patterns 212, 214 and 216 in the time slice associated with the Full 3D pattern 200 are also substantially circular, this is not the case for the three scatterer patterns 312, 314 and 316 associated with the time slice for the cross spread pattern 300, which are substantially less circular. The above-referenced article by Meunier shows that this pattern modification of the scattered waves, i.e., the “non-circularity”, decreases the 3D fk filtering efficiency for the scattered waves.
Accordingly, it would be desirable to provide methods, modes and systems for filtering scattered surface waves, which otherwise obscure useful data, when using a cross spread design in land-based seismic data acquisition.